1. Field of the Invention
The present invention relates generally to storage devices in data processing systems for storing data. Still more particularly, the present invention relates to monitoring a current state of degradation of storage media for managing a data archive.
2. Description of the Related Art
Effective data archiving requires long lifetime media. Media degradation can result from multiple factors: physical wear associated with data transfer mechanisms requiring direct physical contact between data storage device and media such as in tape; media intrinsic degradation mechanisms, such as diffusion, chemical decomposition of functional constituents; manufacturing defects that occurred during the manufacturing of the media; and external environmental factors, such as temperature or relative humidity. The effect of any of these factors on the media's “health” can be very complex and difficult to predict.
Today, media “health” is primarily monitored by reading back the stored data and watching for an increase in error rate that occurs in response to some physical change in the media. As bit error rate (BER) is to some degree a measure of data integrity and its increase a measure of a degradation effect that has already occurred, this may not always provide early enough warning to migrate the data from degraded media in order to prevent loss. Furthermore, as data ages, frequency of access (or intervals between successive data readout operations) decreases (increases), thus limiting the effectiveness of BER monitoring for preventing data loss.
Optical media have been shown to be capable of very long lifetimes given proper handling and controlled storage conditions.
One form of optical data storage is holographic data storage. One major obstacle to its commercial development for many years was the absence of a suitable storage medium. Low-cost, photopolymer-based holographic data storage media having the requisite properties for high density storage have recently been introduced. Among these properties, a key property for high density storage and retrieval is the dimensional stability of the medium with respect to the recording process. Holographic storage media lifetimes are predicted to be 50-100 years, based on initial results from highly accelerated lifetimes. Accordingly, holographic data storage products are considered as a viable alternative for archive data storage.
Holographic recording is achieved by illuminating a photosensitive medium with intersecting reference and object light beams. The spatial modulation of light intensity produced by interference of the beams is recorded in a holographic data storage medium by modification of the dielectric properties of the medium, either in the form of periodic spatial modulation of the refractive index of the medium or of the absorption of the medium, to constitute a grating or a hologram.
One implementation of holographic data storage comprises volumetric digital page holographic storage which allows a large amount of data to be recorded in parallel in the form of a 2-dimensional bit array or data page. This is accomplished by placing a spatial light modulator in the optical path of the object light beam. The spatial light modulator imparts a data page on the object light beam by modulating the spatial profile of the object beam. Each stored data page typically comprises thousands to millions of data bits which are written and read in a single step.
In a high density data storage scheme, the object beam is focused by a focusing lens within the recording medium and recorded as a volume hologram. Volumetric holographic data storage processes, commonly designated as “multiplexing”, can achieve high storage density by recording a large number of page holograms within the same area of the data storage medium. The multiplexing can be achieved by various methods, one of which is angle multiplexing, in which the angle of incidence of the reference beam is changed between successive hologram recordings. By illuminating the holographic data storage medium with an appropriate reference beam, a single associated data page stored in the data storage medium can be reconstructed.
FIG. 1 schematically illustrates a known holographic data storage and retrieval system to assist in explaining the present invention. The system is generally designated by reference number 100 and includes a single laser source 102. Light from laser source 102 is collimated by collimating lens 104, and the collimated light beam from lens 104 is split into two light beams by a polarizing beam splitter (PBS) 106.
Object beam 108 transmitted by polarizing beam splitter 106 impinges on spatial light modulator (SLM) 110, comprising a 2-Dimensional pixel array, which inscribes a data page on object beam 108 by amplitude modulation of the spatial profile of the object beam. Lens 112 focuses the modulated object beam inside holographic data storage medium 114.
Reference beam 116 reflected by PBS 106 is directed by mirrors 118 and 120 onto scanning mirror 122. Lenses 124 and 126 function as a telescope to adjust the size of the reference beam. Scanning mirror 122 deflects the reference beam which then passes through a lens system comprised of lenses 130 and 132. Lenses 130 and 132 keep the reference beam incident on the same location of holographic data storage medium 114 as object beam 108 but with a different angle of incidence determined by the deflection angle of scanning mirror 122.
As shown in FIG. 1, data is recorded in holographic data storage medium 114 as a Fourier hologram. Upon readout of a holographically stored data page by a reference beam with an appropriate angle of incidence, the spatially modulated object beam is reconstructed and collimator 140 directs and forms an image of the retrieved data page upon a 2-dimensional photodetector array 142.
Dimensional stability of the storage medium is critical for data readout. Photopolymer material is known to be sensitive to environmental changes; in particular, it may experience swelling due to moisture absorption. Accordingly, efficient environmental sealing of the photopolymer recording layer is essential for preserving data integrity and readability.
For the most effective management of a long term media library, in order to minimize data loss and reduce the overall cost of ownership of the stored data, it would be desirable to have a method of monitoring the precursors to the media's actual functional degradation. Such a method is especially needed by photopolymer holographic media where the dimension stability of the media is critical to data reading and the media is subject to “swelling” due to moisture absorption. Other conventional optical media types, such as multi-layered, blue laser-based optical media would also benefit from such a method.